High-speed and low-power consumption has become the mainstream in modern communication technology. In many electronic equipment applications, the speed and precision of an Analogue-Digital Converter (ADC) is a key, potentially-limiting performance factor. Further, the performance of a voltage comparator in the ADC has a decisive impact on the overall conversion speed and precision of the ADC. Therefore, the design of the comparator is a key component of the design of the ADC.
In the conventional art, the tail current of the first stage output of a comparator is fixed. Thus, the voltage conversion rate of the first stage output can be limited, wherein the conversion rate indicates the time needed for generating conversion for the output state of the comparator and is generally required to be short as far as possible so as to implement high-speed comparison.
FIG. 1 shows an internal circuit schematic diagram of a comparator according to the conventional art. As shown in FIG. 1, node PGate is a high-potential node of the first stage (namely, transconductance circuit, marked as “GM” in FIG. 1) output of the comparator, and node NGate is the low-potential node of the first stage output of the comparator. FIG. 2 shows a schematic diagram of the output voltage waveform for the node PGate of the first stage output of the comparator according to the conventional art. FIG. 3 shows a schematic diagram of the output voltage waveform for the node NGate of the first stage output of the comparator according to the conventional art. As shown in FIG. 2 and FIG. 3, the node voltage swing of the first stage output of the comparator is always between VGND and VCC, namely, the voltages are in full-swing output.
However, because the swing of the first stage output voltage of the comparator is often directly proportional to the delay time of the comparator, in the conventional art, the full-swing output voltage of the first stage of the comparator can cause a relatively longer delay time of the comparator and thus cause a low voltage conversion rate.